A Treatise on Diamonds, and Precious Stones/Appendix
APPENDIX.
APPENDIX.
IMITATIONS OF COLORED STONES.
THE pink variety of the Topaz being considered the most valuable and beautiful, many impositions have been practised in making imitations of it. The most common is to form a doublet of a white topaz, and to introduce a little color between the pieces. When this manufactured gem is finely cut and close set, it bears a strong resemblance to that which it is intended to imitate; nor will the measures generally adopted to detect fraud in other stones avail here, as, being a topaz, it is inferior neither in hardness nor in lustre to the real gem. If the jewel is close set, and the table very low, this imposition can only be discovered by drawing the stone from its setting, when it will appear perfectly limpid, except in the part where the color is introduced: but if the table is high the fraud may be detected by holding the jewel horizontally between the eye and the light, as the part exposed will then appear perfectly diaphanous.
Similar impositions are practised with regard to the emerald, by covering green glass with a plate of white topaz. This, as well as all other imitations of colored stones set in jewellery, formed by means of the white topaz, may be discovered by the methods above described.
Paste, in which the color is uniform, made to imitate precious stones, may be detected by its being easily abraded by the file, which has no effect upon the real gem.
ON THE DISTINCTIVE CHARACTERS OF THE SPINELLE RUBY.
THE Spinelle Ruby differs from the Oriental in two very decided characters: its color and its crystalline form.
The color of the Spinelle is, as we have already stated, a full carmine red, but it never presents that rich mellow tinge, which attends the oriental. The inferior specimens vary considerably in intensity of color, but do not exhibit the angular lines and opalesceut appearance, which are so generally observed in inferior oriental rubies.
The Spinelle usually occurs in well defined crystals, and frequently presenting the primitive form, a regular octahedron; which is sometimes truncated on the edges and angles. The oriental, on the contrary, is very rarely found crystallized, but generally in rounded fragments.
The inferior hardness and lower specific gravity of the Spinelle, are also very decided marks of distinction, and are easily applicable to the discriminating the two gems when cut and polished.
The Spinelle receives its color from the oxide of chrome, and the oriental ruby from oxide of iron.
From the red topaz, the Spinelle may be distinguished by its very feeble electricity; and from the garnet by its superior hardness and lower specific gravity.
THE OPAL.
MANY impositions have been practised in forming imitations, or enhancing the effect, of the Opal. The first I shall describe (if indeed that can be called an imposition, which, like the shades of a picture, tends to display to greater advantage its rich and glowing colors) is effected by warming the stone, and immersing it in oil or grease, which is afterwards burnt off. The rents which had absorbed the grease, by this means become dark, and agreeably contrast with the beautiful iridescence of the stone, which now assumes the name of the black or green opal.
A variety of opal has lately been imported from Mexico, in considerable quantities. It is pellucid and so soft that it may be scratched by the knife. Some specimens have brilliant patches dispersed in an earthy substance; these appear to great advantage after having been submitted to the process before described.
Grass that has been long exposed to the action of the atmosphere, especially in stagnant waters, often becomes beautifully opalescent, and presents a richness of color equal to that of the gem itself. The cartilage of the hinge of the Chama Gigas, after desiccation, bears also a striking resemblance to the most beautiful varieties of opal, for which, as wellas the preceding, it has sometimes been sold. These imitations may generally be detected by their inferiority in hardness.
TABLE of the Specific Gravity, Primitive Form, Electricity, and Refraction of Gems.
Arranged according to their relative hardness.
| NAMES OF GEMS. | SP. GR. | PRIMITIVE FORM. | ELECTRICITY BY FRICTION. | REFRACTION. |
| Diamond | 3.5 | Octahedron | Feebly, 10 minutes | Slightly double |
| Sapphire | 4.to4.2. | Slightly acute Rhomb | Strongly[1] | Idem |
| Chrysobyrel | 3.8. | Parallelopipedon | Feebly | Idem |
| Spinelle[2] | 3.7 | Octahedron | Feebly | Single |
| Emerald | 2.8 | Hexagonal Prism | Feebly | Double |
| Topaz | 3.5 | Rhomboidal Prism | Very strongly[3] | Double |
| Aquamarine | 2.6 | Hexagonal Prism | Feebly | Slightly double |
| Hyacinth & Zircon | 4.4 | Parallelopipedon | Strongly | Double,in a high degree |
| Tourmaline | 3.2 | Obtuse Rhomb | Strongly | Double |
| Garnet | 4 to 4.4 | Rhombl. Dodecaedron | Very strongly[4] | Single |
| Rock Crystal, Amethyst Yellow Crystal, &c. | 2.5 to 2.7 | Obtuse Rhomb | Feebly | Slightly double |
| Opal | 2 to 2.3 | |||
| Peridot | 3.3. | Parallelpipedon | Feebly | Double |
| Turquois | 2.4 |